The present invention relates generally to diagnosis for an internal combustion engine, and more particularly to a diagnostic apparatus for diagnosing normality of an exhaust purification system of an internal combustion engine.
In recent years, in the field of automotive internal combustion engines, it is desired to improve techniques of exhaust gas purification, especially to improve a technique of exhaust gas purification in cold start. In order to rapidly activate a catalyst, a known device or method is configured to perform a catalyst activation operation of increasing an intake air quantity, and retarding an ignition timing. In addition, it is desired to diagnose the catalyst activation operation. A Published Japanese Patent Application No. 2001-132526 (hereinafter referred to as “JP2001-132526”) shows such a diagnostic device. The diagnostic device of JP2001-132526 is configured to perform a catalyst activation operation of increasing an intake air quantity, and retarding an ignition timing, to monitor the engine speed and the ignition timing after a predetermined delay from a time point at which the catalyst activation operation starts, and to determine that there is a malfunction in the catalyst activation operation, if the engine continues over a predetermined period to be in a condition in which the engine speed is lower than or equal to a predetermined value, or the ignition timing is later than or equal to a predetermined timing.